1. Field of the Invention
The present invention relates to the field of easy-to-open containers and receptacles, and more specifically to easy-to-open metal lids.
2. Description of Related Art
Easy-to-open lids have a generally peripheral score, as in the case of containers for food products--illustrated in FIGS. 1a to 1c. FIGS.1a and 1b correspond to easy-to-open lids with a counter-dish. Alternatively, easy-to-open lids have a score defining a weak portion of the panel of the lid, as in the case of a drinks can lid.
The scores are defined by their profile and the thickness of residual metal under the score, which is also called the "residual".
A profile is defined by the angle alpha of the score and by the calculated width P at the bottom of the score. A typical score is shown diagrammatically in FIG. 2 together with its angle alpha and the width P, a score in which the angle alpha is equal to 60.degree. and the width P is 40 .mu.m being denoted symbolically by "60P40 ".
There is an ever-increasing demand for easy-to-open lids having simultaneously a greater ease of opening and a greater degree of security in production and use. However, these demands are at the present time to a large degree contradictory insofar as a greater ease of opening is generally obtained by reducing the residual. This reduction is however accompanied by much less production security (accuracy of larger tools, smaller safety margin), and above all by the very serious risk of cracks developing at the bottom of or underneath the score, resulting in leakages either directly, or by the action of pressure during packaging, typically involving sterilisation, or after packaging, due to the corrosive action of the packaged product.
Furthermore, the presence of weak residuals and/or cracks makes the lids more susceptible to perforations caused by shock during handling of the packaged products, in particular from the packaging of the product up to its final use by the consumer.
FIG. 7 illustrates a cross-section of a score after crimping and sterilisation, showing at the bottom of the score typical cracks that the present invention aims to suppress if not entirely, then at least to a very great extent. FIG. 7 also shows starting points or foci of cracks underneath the score.
The problem is to find a way of reliably and economically manufacturing easy-to-open lids having a weak residual in which the bottom of the scores does not exhibit cracks at the end of the production stage nor after packaging of the product being packed, so as to present, despite a weak residual, a satisfactory behaviour to mechanical stresses and corrosion.